Geomagnetic Observatory Data Research Articles

Ionospheric current source modeling and global geomagnetic induction using ground geomagnetic observatory data

AbstractLong‐period global‐scale electromagnetic induction studies of deep Earth conductivity are based almost exclusively on magnetovariational methods and require accurate models of external source spatial structure. We describe approaches to inverting for both the external sources and three‐dimensional (3‐D) conductivity variations and apply these methods to long‐period (T≥1.2 days) geomagnetic observatory data. Our scheme involves three steps: (1) Observatory data from 60 years (only partly overlapping and with many large gaps) are reduced and merged into dominant spatial modes using a scheme based on frequency domain principal components. (2) Resulting modes are inverted for corresponding external source spatial structure, using a simplified conductivity model with radial variations overlain by a two‐dimensional thin sheet. The source inversion is regularized using a physically based source covariance, generated through superposition of correlated tilted zonal (quasi‐dipole) current loops, representing ionospheric source complexity smoothed by Earth rotation. Free parameters in the source covariance model are tuned by a leave‐one‐out cross‐validation scheme. (3) The estimated data modes are inverted for 3‐D Earth conductivity, assuming the source excitation estimated in step 2. Together, these developments constitute key components in a practical scheme for simultaneous inversion of the catalogue of historical and modern observatory data for external source spatial structure and 3‐D Earth conductivity.

Conductivities consistent with Birkeland currents in the AMPERE‐driven TIE‐GCM

AbstractThe Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellite mission has offered for the first time global snapshots of the geomagnetic field‐aligned currents with unprecedented space and time resolution, thus providing an opportunity to feed an acknowledged first‐principles model of the Earth's upper atmosphere such as the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (NCAR TIE‐GCM). In the first step, Marsal et al. (2012) used AMPERE data in the current continuity equation between the magnetosphere and the ionosphere to drive the TIE‐GCM electrodynamics. In the present work, ionospheric conductivities have been made consistent with enhanced upward field‐aligned currents, which are assumed to correspond to electrons plunging as a result of downward acceleration by electric fields built up along the geomagnetic field lines. The resulting conductance distribution is reasonably commensurate with an independent model that has tried to quantify the ionizing effect of precipitating particles onto the auroral ionosphere. On the other hand, comparison of geomagnetic observatory data with the ground magnetic variations output by the model only shows a modest improvement with respect to our previous approach.

Valuation of induction current contribution to the secular variation of geomagnetic field (according to the data of Ukrainian geomagnetic observatories)

Induction current contribution to the secular variation of geomagnetic field in accordance with the data of Ukrainian geomagnetic observatories has been investigated. The dynamics of the monthly changes of the secular variations depending on the values of the induction currents has been shown. Correlation between changes of secular variations and solar activity has been found. Changes of solar activity outstrip secular variation changes.

Three-dimensional electrical conductivity structure beneath Australia from inversion of geomagnetic observatory data: evidence for lateral variations in transition-zone temperature, water content and melt

In this paper, we report the 3-D electrical conductivity distribution beneath the Australian continent in the depth range 410–1600 km, which we have imaged by inverting C-response estimates from a regional network of geomagnetic observatories. The inversion scheme is based on a quasi-Newton optimization method while the forward algorithm relies on an integral-equation approach. To properly account for the ocean effect in responses at coastal observatories we included a high-resolution (1° × 1°) fixed thin laterally varying surface conductance layer. As starting model in the inversion we considered a laboratory-based 3-D conductivity model of the region obtained from seismic surface wave data and thermodynamic modelling. This model provides a good fit to observed C-response estimates supporting its choice as initial model. The most striking feature of the obtained 3-D model is a high-conductivity anomaly in the lower part of the mantle transition zone (MTZ; 520–660 km depth) beneath southeastern Australia implying considerable lateral as radial heterogeneity in the conductivity structure. The high-conductivity region appears to be 0.5–1 log units more conductive than previous global and other regionalized 1-D models. Further analysis using laboratory-based conductivity models combined with thermochemical phase equilibrium computations shows that the strong conductivity anomaly implies water contents of around 0.1 wt per cent in the upper part and >0.4 wt per cent in the lower part of the MTZ. This implies a large MTZ water reservoir that likely totals one to three times that which currently resides in the oceans. The amount of water in the lower MTZ appears to exceed the experimentally determined water storage capacity of the main lower MTZ mineral ringwoodite, which, as a result, undergoes dehydration-induced partial melting. Including contributions to conductivity from a thin melt layer (20 km thick) located in the mid-MTZ increases conductivity locally in the melt layer to ∼1 S m−1, that is, about 0.5 log units more conductive than the average surrounding mantle. This provides an adequate explanation for the strong conductivity anomalies observed beneath part of the continent and points to lateral variations in melt in the MTZ.

Jerks abound: An analysis of geomagnetic observatory data from 1957 to 2008

We present a two-step method for the removal of external field signals and the identification of geomagnetic jerks in magnetic observatory monthly mean data, providing quantitative uncertainty estimates on jerk occurrence times and amplitudes with minimal a priori information. We apply the method to the complete time series of X-, Y- and Z-components at up to 103 observatory locations in the period of 1957–2008. We find features fitting the definition of jerks in individual components to be frequent and not globally contemporaneous. Identified regional jerks have no consistent occurrence pattern and the most widespread in any given year is identified at <30% of observatories worldwide. Whilst we identify jerks throughout the period of study, relative peaks in the global number of jerk occurrences are found in 1968–71, 1973–74, 1977–79, 1983–85, 1989–93, 1995–98 and 2002–03 with the suggestion of further poorly sampled events in the early 1960s and late 2000s. The mean uncertainties on individual jerk occurrence times and amplitudes are found to be ±0.3yrs and ±2.1nT/yr2, respectively, for all field components. Jerk amplitudes suggest possible periodic trends across Europe and North America, which may be related to the 6-yr periods detected independently in the secular variation and length-of-day.

Variation of the components of the induction vector and the horizontal tensor before the Tohoku earthquake March 11, 2011 according to the data of Japanese geomagnetic observatories

This work is a continuation of a work [Babak et al., 2013]. The sources of variable magnetic field observed at the surface of the Earth are presented in the introductory part. The most reliable cases of lithospheric emission registered before strong earthquakes are described for five seismic regions of the World. The changes of electrical conductivity of the Earth's crust as a precursor of earthquakes are discussed. The review shows that geomagnetic response functions (induction vector and horizontal tensor) can be used as an earthquake precursor, which carry information both on lithospheric emission and changes in the electrical conductivity. Analysis of 18 Japanese geomagnetic observatories data (6 of them are away from the Tohoku earthquake epicenter less than 400 km) shows the presence of intense noise for periods less than 500 s. Application of noiseproof algorithms for calculation of response function components only partially reduced their influence. A large amount of processing was carried out and database of response functions for 10-20 years for 18 Japanese observatories was created. During the processing variety of record deficiencies were found, the cause of which is not always clear. Detailed processing of one-second data for 10 weeks before the Tohoku earthquake reveals anomalies of response functions temporal behavior: 7-9, 19-21, 33-36 and perhaps ~ 60 days before the event. Moments of amplitude and phase anomalies vary slightly for different periods of variations, for different components of the response functions, for different stations. This behavior is caused, on one hand, by the influence of geomagnetic activity and the contribution of noise, on the other hand, it can be the manifestation of precursors and co-seismic events, and this behavior may reflect a migration of lithospheric emission and/or changes in the electrical conductivity of the Earth's lithosphere. Decoding of this information can provide valuable information for understanding the physics of earthquake preparation and precursors formation.

Repeat station data compared to a global geomagnetic field model

&lt;p&gt;Geomagnetic repeat station surveys with local variometers for improved data reductions have been carried out in Germany for about ten years. For nearly the same time interval the satellites Ørsted and CHAMP have provided a good magnetic field data coverage of the whole globe. Recent global field models based on these satellite data together with geomagnetic observatory data provide an improved description of the core field and secular variation. We use the latest version of the GFZ Reference Internal Magnetic Model to compare the magnetic field evolution predicted by that model between 2001 and 2010 to the independent repeat station data collected over the same time interval in Germany. Estimates of crustal bias at the repeat station locations are obtained as averages of the residuals, and the scatter or trend around each average provides information about influences in the data from field sources not (fully) described by the global model. We find that external magnetic field signal in the order of several nT, including long-term trends, remains both in processed annual mean and quiet night time repeat station data. We conclude that the geomagnetic core field secular variation in this area is described to high accuracy (better than 1 nT/yr) by the global model. Weak long-term trends in the residuals between repeat station data and the model might indicate induced lithospheric anomalies, but more data are necessary for a robust analysis of such signals characterized by very unfavorable signal-to-noise ratio.&lt;/p&gt;

A new versatile method for modelling geomagnetic induction in pipelines

S U M M A R Y Geomagnetic induction drives telluric currents in pipelines and creates fluctuations in pipe-tosoil potentials (PSP) that interfere with pipeline surveys and create conditions where corrosion is more likely to occur. To understand the process of geomagnetic induction and determine the severity and location of troublesome effects requires the ability to model geomagnetic induction in realistic pipeline networks. Previous modelling work, based on transmission line theory, has provided some insights into the process but has to be customized for each situation. This paper presents a new versatile modelling technique that can be easily applied to any pipeline network. The essential part of the new method is the development of an equivalent-pi circuit for geomagnetic induction in a pipeline section. A complex pipeline network can then be represented as a set of equivalent-pi circuits that are combined to form a nodal admittance network comprising connections between nodes and to ground from each node. The nodal admittance matrix method is then used to determine the voltages everywhere in the pipeline system. Sample results are presented for geomagnetic induction in an example pipeline. It is shown how the modelling results can be combined with electric fields calculated from geomagnetic observatory data to determine the PSP variations that occur during geomagnetic disturbances.

Modeling of deep magnetovariation soundings on the rotating earth

Induced magnetic fields in the Earth arise due to two phenomena: induction generated by the time-variable exciting field and the motional induction caused by movement of the conductive planet in the outer magnetic fields. The comparison of both approaches on the spherical Earth has been analyzed in the present work for two sources in the ionosphere and magnetosphere. For this aim, both sources with their natural sizes and positions have been modeled analytically to obtain the fields on the layered sphere at the middle latitudes. The conditions when the steady ring current field is not influenced by the Earth’s rotation have been established theoretically. The synthetic diurnal magnetograms were used for the deep sounding by the magnetovariation spatial gradient method and the result was compared with the one obtained on the nonrotating sphere. Sounding results using both approaches were found different above the 2D inhomogeneous mantle. The precessions of the magnetospheric belt current pole for daily sampling frequency were presented using several geomagnetic observatory data in the northern hemisphere.

ГЕОДИНАМІКА

The spatial structure of geomagnetic field secular variations in Europe was examined. Using the data of geomagnetic observatories the new maps of this secular variation (SV) for X, Y, Z, D – components for 5-years intervals 1995-2000 and 2000-2005 were constructed. Their comparison with the IGRM global geomagnetic models was done. The global anomalies – SV-focuses in the structure of the secular variations in Europe were discovered and their kinematics was studied.

New induction sounding tested in Central Europe

The hourly data of nine geomagnetic observatories situated in Central Europe have been analyzed using the generalized magnetovariation (GMV) method designed recently for induction soundings of inhomogeneous media. In this method, impedance is one of transfer functions in the differential relation between spectra of the magnetic components and their derivatives. The peculiarity of this impedance is its correspondence to the magnetotelluric one estimated from the linear relations. Three transfer functions have been estimated simultaneously for data of geomagnetic observatories, using three different routines working in the period range from three hours up to two days. Noises in the source field components have been compared with noise in the estimated plane field divergence. The multivariate errors-in-variables method was used to extract spatially and temporally coherent geomagnetic field structure from the partially incoherent geomagnetic variations. This method allows estimating reliably impedances and gradient tippers for each observatory, taking into consideration the Earth’s sphericity. The obtained responses have been used for induction soundings and for detecting a deep inhomogeneity in the region.

On the heterogeneous electrical conductivity structure of the Earth’s mantle with implications for transition zone water content

[1] We have investigated the lateral variations in mantle electrical conductivity structure using electromagnetic sounding data. For this purpose, we used very long time series (up to 51 years) of geomagnetic observatory data at six locations encompassing different geological settings to compute response functions that cover the broadest possible frequency range (3.9 to 95.2 days): Furstenfeldbruck (FUR), Europe; Hermanus (HER), South Africa; Langzhou (LZH), China; Alice Springs (ASP), Australia; Tucson (TUC), United States (North America); and Honolulu (HON), United States (North Pacific). We inverted the response functions beneath each observatory for a local radial conductivity profile using a stochastic sampling algorithm. Specifically, we found significant lateral variations in conductivity throughout the mantle with resolution limited to the depth range ∼500–1200 km. At 600 km depth, conductivity varies between 0.1 and 0.4 S/m and increases to 1.3–2.0 S/m at 800 km depth beneath all stations except HER (0.5 S/m). At 900 km depth, conductivity increases further to 1.4–2.4 S/m with HER, HON, and ASP being most conductive. This trend persists to a depth of 1200 km. Comparison with conductivity profiles constructed from laboratory measurements of mantle mineral conductivities and models of Earth’s mantle composition and thermal state reveal that significant thermochemical variations are at the origin of the observed heterogeneities in mantle conductivity found here. Because of the somewhat large error bounds on sampled conductivity profiles and the reduced sensitivity of the electromagnetic sounding data above 500 km depth, constraints on transition zone water content are less conclusive, although H2O contents <0.5 wt% in the midtransition zone appear less likely.

Proceedings of the XIV IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition, and Processing

The CODATA&nbsp;Data Science Journal&nbsp;is a peer-reviewed, open access, electronic journal, publishing papers on the management, dissemination, use and reuse of research data and databases across all research domains, including science, technology, the humanities and the arts. The scope of the journal includes descriptions of data systems, their implementations and their publication, applications, infrastructures, software, legal, reproducibility and transparency issues, the availability and usability of complex datasets, and with a particular focus on the principles, policies and practices for open data.All data is in scope, whether born digital or converted from other sources.

Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling

Summary This paper describes updated uncertainties for use with predicted geomagnetic parameters within magnetic measurement-whiledrilling (MWD) survey-tool-error models. These models are used to define positional-error ellipsoids along the wellbore, which assist in hitting geological targets and avoiding collisions with existing wellbores. The declination, dip angle, and total field strength of the Earth's magnetic field are used with magnetic-survey tools for surveying the wellbore. These values are often obtained from mathematical models such as the British Geological Survey (BGS) global-geomagnetic model (BGGM). As the Earth's magnetic field is continually varying with time, the BGGM is updated annually to maintain accuracy. However, a global predictive model cannot capture all sources of the Earth's magnetic field, which results in uncertainties of the predicted parameters. The Industry Steering Committee on Wellbore Surveying Accuracy (ISCWSA) published an MWD-error model in 2000 (Williamson 2000). The geomagnetic-field uncertainties that are part of this model were derived from work conducted by the BGS in the early 1990s. Since then, more-accurate data from magnetic-survey satellites have been introduced into the BGGM, and the uncertainty of the predicted geomagnetic-field parameters has been reduced. The original approach to deriving the uncertainties involved separating the various error sources in the magnetic field and assessing them individually. This paper uses a simpler approach where clean orientated magnetic downhole data are simulated using geomagnetic-observatory data. Spot absolute measurements of the magnetic field made at observatories around the world are adjusted for the crustal magnetic field to make them more representative of hydrocarbon geology. The adjusted observatory data are then compared with the predicted values from the BGGM to assess the uncertainty. The uncertainties do not fit a normal distribution, so they are expressed as limits for various confidence levels. They vary with location and, in their derivation, do not assume any underlying empirical error distribution. While they also vary with time, we provide time-averaged look-up tables that should be valid for as long as there are good-quality satellite data on which to base global magnetic-field models. Options to reduce the uncertainties further using data from local magnetic surveys [in-field referencing (IFR)] and observatories (interpolation IFR) are also described. The use of the revised geomagnetic uncertainty values in the MWD-error model will reduce wellbore-position uncertainty to reflect the increased accuracy from recent improvements in geomagnetic modeling. This is demonstrated using results for the ellipsoids of uncertainty output by an MWD error model for three standard ISCWSA well profiles.

Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 119851, "Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling," by Susan Macmillan, British Geological Survey; Allan McKay, Petroleum GeoServices; and Steve Grindrod, SPE, Copsegrove Developments, prepared for the 2009 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 17-19 March. The paper has not been peer reviewed. Uncertainties have been updated for use with predicted geomagnetic parameters in magnetic measurement-while-drilling (MWD) survey-tool-error models. These models define positional-error ellipsoids along the wellbore that assist in hitting geological targets and missing existing wellbores. The declination D, dip angle I, and total field strength F of the Earth's magnetic field are used with magnetic-survey tools for surveying the wellbore. The use of revised geomagnetic-uncertainty values in the MWD-error model could reduce wellbore-position uncertainty. Introduction The Earth's magnetic field is a vector quantity that depends on position and time and may be expressed as the vector sum of three sources: the field generated in the Earth's core, the crustal field from local rocks, and a combined external field from electrical currents flowing in the upper atmosphere and in the magnetosphere. Global models used in directional drilling [e.g., the British Geological Survey Global Geomagnetic Model (BGGM)] generally represent a combination of the main (i.e., internal) field and the undisturbed external field arising from ever-present magnetospheric currents. The values computed for the Earth's magnetic field at a given location and time from the BGGM will differ from actual values because of a combination of model errors, crustal fields, and external fields. This study investigated the magnitude of the uncertainty. Data and Analysis Directional-drilling and survey-management companies require accurate geomagnetic-field estimates, along with knowledge of associated uncertainties, when magnetic-survey tools are used to survey wellbores. Magnetic-survey tools measure the direction of the wellbore relative to the direction of the local geomagnetic field. In addition, the geomagnetic-field I and F are required for use in algorithms that reduce the error caused by the magnetic field associated with the drillstring. The uncertainties are used to compute positional-error ellipsoids along the wellbore to help intercept increasingly small geological targets and miss other wellbores. Geomagnetic-observatory data were used to estimate BGGM uncertainty. The difference was computed between model predictions and observatory data not used in the construction of a given BGGM. A "BGGM year" is the year starting soon after the release of a BGGM in May of each year. For example, the 2006 BGGM year spans 2006.5 to 2007.5. The majority of dates entered for magnetic-field computations that use a given release of the BGGM are expected to be within this period. The overall performance of BGGM revisions has changed over the years.

Non-linear conjugate gradient inversion for global EM induction: resolution studies

SUMMARY We develop a non-linear conjugate gradient inversion for global long period electromagnetic induction studies. The scheme requires computation of derivatives of the regularized penalty functional. We derive analytical and numerical expressions for these derivatives, and the associated Jacobian, and show how these can be efficiently implemented by generalizing and extending an existing finite difference forward solver. Using layered spherical harmonics to parametrize the model space, we invert a range of synthetic data sets to test the inversion, and to study vertical and horizontal resolution of currently available data sets. We conclude that the currently available long-period global geomagnetic observatory data in the period range 5–107 d can resolve large scale (300–500 km vertically, thousands of km horizontally) heterogeneities in mantle electrical conductivity reliably at depths ∼ 670–1600 km. By extending induction response to 0.2–5 d (including daily variation periods), upper-mantle structure could also be resolved.

Completing the Spectrum of the Australian Digital Magnetic Anomaly Map

Continental-scale merges of Australian airborne magnetic data are only accurate for wavelengths less than about 100 km, due to limitations of survey size and data processing. Wavelengths greater than 400 km are available from satellite data; thus, there is a ``gap' in the spectrum for intermediate wavelengths between 100 km and 400 km. Geoscience Australia is filling this gap by acquiring new airborne magnetic data as part of the Australian Government?s new energy initiative. Intermediate wavelengths are important, for example, for better definition of sedimentary basins for petroleum prospectivity evaluation, for interpreted depths to bottom of magnetic sources in attempts to define the Curie point isotherm, and for regional removal in modelling. The AWAGS2 project (Australia-wide Airborne Geophysical Survey) is collecting both gamma-ray spectrometric and total-field magnetic data across Australia with long north-south lines spaced 75 km apart and 80 m terrain clearance. Accurate spatial crustal magnetic data depend upon accurate removal of time variations of Earth?s magnetic field. These are recorded at pairs of ground sites, concurrently with the airborne acquisition, and supplemented by geomagnetic observatory and other data. Routine airborne magnetic surveys rarely use more than one base magnetometer and the long lines of AWAGS2 present a challenge. It cannot be assumed that time variations are spatially uniform (eg., induction in the oceans and crust creates spatial non-uniformities). Removal of time variations depends upon interpolation of data recorded at ground sites and knowledge of induction effects. Removal of the latter is aided by using high-resolution vector data recorded at the base sites. Using the AWAGS2 corrected traverses will improve the accuracy of intermediate wavelengths in the Australian Digital Magnetic Anomaly Map.

Methods and results of monitoring of the natural telluric field in the Baikal rift zone

Variations in horizontal and vertical components of the telluric field are studied within the range of periods from a few tens of seconds to diurnal and longer variations. Variations caused by an external ionospheric source are used for studying the geoelectric heterogeneity of the medium. For this purpose, an electric tipper is introduced by analogy with the geomagnetic field and its frequency response is examined. The magnetotelluric impedance is studied using data of remote geomagnetic observatories. According to the data of monitoring of the electric tipper and magnetotelluric impedance, anomalous effects are revealed that can be related to earthquakes with K ≥ 12. A method is developed for the extraction of electric field anomalies produced by intraterrestrial electric sources.

Empirical orthogonal function analysis of magnetic observatory data: Further evidence for non‐axisymmetric magnetospheric sources for satellite induction studies

Although satellite electromagnetic induction studies have usually assumed a symmetric magnetospheric ring current source, there is growing evidence for significant source asymmetry. Here we apply empirical orthogonal function methods to mid‐latitude night‐side hourly mean geomagnetic observatory data to search for evidence of non‐zonal low‐frequency source fields. The dominant spatial mode of variability in residuals, obtained by subtracting symmetric ring current and ionospheric fields of the CM4 comprehensive model, has a substantial Y2−1 quadrupole component and is highly correlated with Dst. This pattern of temporal variability, which implies enhanced ring current densities in the dusk sector, persists even when peak storm‐time data are omitted. The observed asymmetry agrees with that inferred previously by Balasis et al. (2004), from the local time dependence of biases in satellite induction transfer functions. Temporal correlation of the leading mode with Dst, and consistency of its spatial structure with recent empirical ring current models, suggest a magnetospheric origin.

Long-term seafloor geomagnetic station in the northwest Pacific: A possible candidate for a seafloor geomagnetic observatory

Abstract For two years, geomagnetic variations have been measured at the seafloor in the northwest Pacific. The seafloor data consist of the geomagnetic vector field measured by a three-component fluxgate magnetometer and the absolute scalar total force measured by an Overhauser (1953) magnetometer with attitude measurements for both orientation and tilt. Using the attitude data, the geomagnetic data at a site in the northwest Pacific (41o06′08″N, 159°57′47″E, -5580 m), hereafter referred to as NWP, were converted into the same reference frame as land and satellite measurements. Short-period variations of the converted vector data were examined by Hamano’s (2002) global time domain analysis method, which showed compatibility of the seafloor geomagnetic observatory data with the existing land observatory network. The smooth and gradual change of the Earth’s main field (i.e., the geomagnetic secular variation) was also found consistent with those predicted by the latest International Geomagnetic Reference Field (IGRF-10; IAGA, 2005) and by Ørsted Satellite (Olsen, 2002) for not only the scalar field but also the vector field. This means that observation of the geomagnetic vector secular variation is now feasible on the seafloor.